1. Background introduction

　　With the continuous development of the electric power industry in the direction of high voltage and large capacity, traditional electromagnetic current transformers based on the principle of electromagnetic induction have exposed more and more problems. Insulation is difficult under high voltage, explosion is prone to occur, measurement range and accuracy are limited, and system is prone to saturation under system failure. In recent years, the rapid development of optoelectronics and optical fiber communication has promoted the research and application of new fiber optic current transformers (Fiber Optic Current Transducer, FOCT).

　　FOCT is based on the Faraday magneto-optical effect. It determines the magnitude of the current by measuring the angle at which the polarization plane of the light wave passes through the magneto-optical material due to the magnetic field generated by the current. Compared with traditional electromagnetic current transformers, FOCT has many advantages, including small size, simple insulation structure, no safety hazards, no magnetic saturation, high measurement bandwidth and accuracy, and anti-electromagnetic interference. It can adapt to the working voltage of the transmission network. The ever-increasing requirements also meet the requirements for high precision, high range, safety and reliability.

2. Working principle

　  The working principle of FOCT is shown in the figure below. The light emitted from the broadband light source passes through the polarizer to obtain linearly polarized light. The linearly polarized light is injected into the polarization maintaining fiber at 45 degrees and propagates forward along the fast axis or the slow axis, respectively. After passing through the quarter-wave plate, the two orthogonal modes become left-handed and right-handed circularly polarized light enter the sensing fiber loop, and are affected by the magnetic field generated by the current in the conductor. The left-right circularly polarized light propagates at different speeds, causing light waves. Phase change. After the light is specularly reflected at the sensor end, the polarization modes of the two circularly polarized light ends are exchanged, and the sensing fiber is again subjected to the magnetic field to double the effect. After the two circularly polarized lights pass through the wave plate, they are restored to linearly polarized light, and they interfere at the polarizer, and the phase difference is detected by measuring the interference light intensity, thereby obtaining the magnitude of the measured current.

Required devices:

SLD（SuperLuminescent Diode）； 
PIN：Photodetector； 
Coupler； 
Polarizer； 
Modulator； 
PM Fiber； 
QWP（Quarter Wave Plater)； 
Mirror； 
Sensing Fiber Loop: ordinary communication fiber； 

Conductor.